During the 1970s, US and UK banks lent large amounts of money to countries in the developing world. Levels of interest on bank loans were very high at the time.
After Mobuto's death, the World Bank decided it would be unfair to expect the desperately poor people of DR Congo to pay back the stolen money. DR Congo's debt was 'written off'.
Debt relief can also be achieved through 'conservation swaps'. A richer country may agree to write off part of a poorer country's debt if that poorer country agrees to protect its physical environment.